With the increasing popularity of HDTV (high-definition television), 3D (three-dimensional) displays, and peer-to-peer file sharing, the demand for high bandwidth connectivity to the home will continue to grow. Fiber-to-the-Home (FTTH) has grown rapidly world-wide and the Asia Pacific market leadership is expected to continue in the next few years. Recently, the passive optical network (PON) system has been well recognized as a potential FTTH system in terms of its offered capacity and cost. Optical transceivers used for the PON system are bi-directional devices that use different wavelengths to transmit and receive signals between the optical line terminal (OLT) at the central office and the optical network units (ONUs) at the end users' premises over a single fiber. Nowadays, there are different approaches to produce a bidirectional optical transceiver including (1) free space packaging by employing TO-CAN laser diode (LD), APD (avalanche photodiode) and thin film filter; (2) planar lightwave circuit (PLC) with discrete optical components including LD, APD and WDM (wavelength-division multiplexing) filter. In order to reduce the number of discrete components, improve the manufacturing yield, increase the reproduction throughput, and reduce overall cost, highly integrated solutions based on hybrid or monolithic integration are demanded.
Due to the recent inspiring developments, silicon photonics has become a promising technology for low cost optical transceivers with high integration density. Waveguides, optical filters, modulators, and photo-detectors can be integrated by CMOS (complementary metal-oxide-semiconductor) compatible processes on a single silicon chip to fulfill the transceiver's functions. Electrical drivers and amplifiers can be furthermore integrated with the silicon photonics circuit on one chip.
For practical application in an optical transmission system, polarization diversity in a PIC (photonic integrated circuit) is normally required. The most common way to realize polarization independent silicon PIC is to implement a polarization diversity scheme. The transverse electric (TE)- and transverse magnetic (TM)-polarized components of the input light will be split into two paths and then converted to a single polarization so that the polarization dependent structures in the PIC will have identical performance for both paths. This can be realized by using a fiber-to-waveguide grating coupler.
As is known, a grating structure is a narrow band structure. The 3 dB optical coupling bandwidth for a silicon grating coupler is in the order of 50-80 nm. In order to satisfy the multiple wavelength band requirement in transceiver applications, a duplexer grating coupler may be used. However, the conventional 2D (two-dimensional) square lattice diffraction grating has its drawbacks as a building block for optical transceivers. Because of the symmetric grating structure, the four access waveguides are equally assigned to two wavelength channels. But the transmitter part with a single polarized LD output would only require a single access waveguide. Such symmetric grating requires the fiber to be tilted along the symmetry axis in order to realize wavelength duplexing operation, which calls for additional effort to match the projected Bragg condition. Furthermore, in practice, the polarization independence behavior only works over a limited wavelength range.